Perovskite NdBaMn2O6 Single Crystal

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Apr 20, 2018 - and that the magnetic anomaly at TMI should be ascribed to layered ... The ferromagnetic fluctuation are also observed just below TMI .
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Successive Phase Transitions and Magnetic Fluctuation in a DoublePerovskite NdBaMn2O6 Single Crystal To cite this article: S Yamada et al 2018 J. Phys.: Conf. Ser. 969 012103

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28th International Conference on Low Temperature Physics (LT28) IOP Conf. Series: Journal of Physics: Conf. Series 969 (2018) 1234567890 ‘’“”012103

IOP Publishing doi:10.1088/1742-6596/969/1/012103

Successive Phase Transitions and Magnetic Fluctuation in a Double-Perovskite NdBaMn2O6 Single Crystal S Yamada1 ,H Sagayama2,3 ,K Sugimoto4 and T Arima5 1 Department of Materials System Science, Yokohama City University, Yokohama, 236-0027, Japan 2 Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan. 3 Department of Materials Structure Science, The Graduate University for Advanced Studies, Tsukuba, Ibaraki 305-0801, Japan. 4 SPring-8/JASRI, Kouto, Sayo, Hyogo, 679-5198, Japan 5 Department of Advanced Materials Science, The University of Tokyo, Kashiwa, 277-8561, Japan

E-mail: [email protected] Abstract. We have succeeded in growing large high-quality single crystals of doubleperovskite NdBaMn2 O6 with c-axis aligned. Curie-Weiss paramagnetism and metallic conduction are observed above 290 K (TM I ). The magnetic susceptibility suddenly drops at TM I accompanied by a metal-insulator transition. Pervious studies using polycrystalline samples proposed that this material undergoes a ferromagnetic phase transition near 300K, and that the magnetic anomaly at TM I should be ascribed to layered antiferromagnetic phase transition. However, single-crystalline samples do not show any anomaly that indicates the ferromagnetic phase transition above TM I . We assign the onset of magnetic anisotropy at 235 K as antiferromagnetic transition temperature TN . Though the magnetization just above TM I shows the ferromagnetic-like magnetic-field dependence, the magnetization does not saturate under 70kOe at 300K. The magnetization behavior implies ferromagnetic fluctuation in the paramagnetic phase. The ferromagnetic fluctuation are also observed just below TM I . Because a metamagnetic transition is observed at a higher magnetic field, the ferromagnetic fluctuation competes with antiferromagnetic fluctuation in this temperature range.

1. Introduction The physical properties of a family of double-perovskite manganites REBaMn2 O6 (where RE is divalent rare earth) were attracting much attention in terms of A-site-randomness free perovskite manganites.[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12] Because magnetic and electric ordering phases are stabilized by the A-site ordering, the phase diagram of REBaMn2 O6 is completely different from that of A-site disordered perovskite manganites RE0.5 Ba0.5 MnO3 , where the magnetic ground state is paramagnetic or spin glass phase. When the ionic radius of RE is equal to or smaller than Sm, charge ordering phase is observed even above 380 K. The highest charge order is observed at 550 K in YBaMn2 O6 . This material has the potential for room temperature magnetoresistance device due to the melting of the charge ordering. Actually, fairly large magnetoresistance at

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28th International Conference on Low Temperature Physics (LT28) IOP Conf. Series: Journal of Physics: Conf. Series 969 (2018) 1234567890 ‘’“”012103

IOP Publishing doi:10.1088/1742-6596/969/1/012103

room temperature was observed in A-site ordered Sm1−x Lax+y Ba1−y Mn2 O6 .[12] On the other hand, when the ionic radius of RE is equal to or larger than Nd, the charge ordering phase is not observed. The previous studies using the NdBaMn2 O6 polycrystal suggested that a ferromagnetic phase transition was observed near 310 K, and A-type antiferromagnetic phase was stabilized below 290 K.[7, 11] Recently, we succeeded in the single crystal growth of a double-perovskite manganite SmBaMn2 O6 , and revealed that the magnetic property was different from polycrystals.[13] Moreover, we found that the spatial inversion symmetry was broken in low-temperature charge ordering phase by means of the single crystal structure analysis using synchrotron x-ray diffraction.[14, 15] These results suggest the importance of the studies using single crystal for double perovskite manganites. We also succeeded in the single crystal growth of NdBaMn2 O6 .[16] The studies on the magnetic, electric and crystal structure using the single crystal provided new information about the phase transitions. In particular, the study on the magnetic anisotropy showed that a steep change of magnetic susceptibility near 290 K (TM I ) was not caused by the antiferromagnetic phase transition and the N´eel temperature was near 235 K (TN ) which was more 50 K lower than TM I . Additionally, we did not clearly observe the ferromagnetic transition near 300 K, which were proposed by previous papers. In general, two dimensional metallic conduction is observed in A-type antiferromagnetic phase[17]. Nevertheless, an insulating behavior was observed not only along the c-axis but also in the ab-plane below TM I . The magnetic phases around TM I might be more complicated than reported by pervious studies. In this paper, we revise the magnetic phase diagram of NdBaMn2 O6 through magnetization measurements using single crystals.

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2. Experimental TN TMI TP 2.0 14 Single crystals of NdBaMn2 O6 were grown by (a) H ^ c 12 H floating zone (FZ) method. Powders of Nd2 O3 , 100 Oe 1.5 10 BaCO3 , and Mn3 O4 were mixed, ground, and 10 kOe 8 ◦ 1.0 calcined at 1290 C for 48 hours in Ar (6N) at6 mosphere, and then pulverized. The resultant 4 0.5 powder was shaped into a cylinder under a hy2 drostatic pressure of 30 MPa and sintered at 0.0 0 (b) H^c 1290 ◦ C for 12 hours in Ar (6N) atmosphere to 0.03 H = 10 kOe form feeding and seeding rods. An FZ furnace H // c equipped with two halogen incandescent lamps and hemielliptic focusing mirrors was used for 0.02 H^c the crystal growth. The molten zone was ver2 10 tically scanned at a rate of 2 mm/h in Ar gas (c) 1 I^c mixed with a tiny portion (≤ 0.1 %) of H2 . The 10 0 melt grown bar was annealed in O2 atmosphere 10 I // c at 500 ◦ C for 48 hours. In order to measure -1 10 anisotropic properties, we cut the crystal boules -2 10 into some pieces, and the orientation of the c-3 axis was determined by x-ray diffraction. Laue 10 150 200 250 300 350 photographs confirmed that every sample was a T (K) single crystal. Magnetization was measured using a commercial superconducting quantum inTemperature dependence of terface device magnetometer (Quantum Design Figure 1. (a) magnetic susceptibility, (b) magnetic MPML-XL). The electrical resistivity was meaanisotropy, and (c) electrrical resistivity.[16] sured by a conventional four-probe method. The solid and open squares in (a) and (b) denote the cooling and warming processes, respectively.

28th International Conference on Low Temperature Physics (LT28) IOP Conf. Series: Journal of Physics: Conf. Series 969 (2018) 1234567890 ‘’“”012103

IOP Publishing doi:10.1088/1742-6596/969/1/012103

M (B/Mn)

3. Results and Discussions Figure 1 shows that the magnetic susceptibility steeply changes around 290 K (TM I ). This magnetic anomaly is accompanied by a metal-insulator transition. Though previous studies using poly-crystal samples suggested that TM I should correspond to A-type antiferromagnetic phase transition temperature, no clear magnetic anisotropy is observed down to 235 K. Therefore, we conclude that N´eel temperature TN of NdBaMn2 O6 is not identical to TM I but 235 K. When an applied magnetic field is 100 Oe, magnetic susceptibility steeply increases with a decrease of temperature around 310 K, as denoted by a large arrow in Fig. 1(a). On the other hand, when an applied magnetic field is 10 kOe. No anomaly is observed above TM I . To reveal the magnetic phase just above TM I , magnetic field dependence of magnetization was investigated in this temperature range. At 350 K which is sufficiently higher than TM I , the magnetization linearly increases with an applied magnetic field, like a typical paramagnet. The magnetization curves become convex when approaching TM I . Below 10 kOe, the magnetization steeply increases with the applied magnetic field. The increase becomes gradual above 20 kOe. The magnetization is smaller than the magnitude of full magnetic moments of Mn3.5+ even when the applied magnetic field is 70 kOe. It suggests that ferromagnetic fluctuation in the paramagnetic phase is steeply enhanced down to TM I . We conclude that the temperature at which the ferromagnetic fluctuation arises is 310 K (TP ) from Weiss temperature in our previous report.[16] Next, we discuss the magnetic phase just 2.0 below TM I . Fig. 3 (a) shows that no magnetic anisotropy is observed below 70 kOe in 1.5 this temperature range. The magnetization jumps near 20 kOe. The isotropic magnetic susceptibility below metamagnetic phase transi1.0 tion and the metamagnetic behavior imply that 300 K the long-range antiferromagnetic ordering is not 305 K 0.5 present but the antiferromagnetically fluctuat310 K 320 K ing domains exist in the magnetic ground state 350 K in this temperature range. The metamagnetic 0.0 0 10 20 30 40 50 60 70 phase transition field depends on the measureH (kOe) ment process. Because the phase transition at TM I is of the first-order, the higher magnetic field phase is essentially identical to that above Figure 2. Magnetic field dependence of TM I . In low magnetic field range, the magneti- magnetization above TM I . zation curve of sequence 3 is ferromagnetic-like and different from that of sequence 1. Fig. 3 (b) shows that, below 1 kOe, the magnetization curve after zero field cooling is different from the others. This magnetic field dependence of magnetization in a low magnetic field suggests that the application of a magnetic field induces a phase transition, and the initial state never revives by further field sweeping. At 270 K, which is a little lower than 286 K but still higher than TN , the metamagnetic transition field shifts to higher one. Additionally, the ferromagnetic-like behavior in low magnetic field decreases. Hence the ferromagnetic fluctuation is suppressed with a decrease of temperature. As shown in Fig. 4, at 150 K which is below TN , the magnetization linearly increases to an applied magnetic field, and the magnetic anisotropy is clearly observed. The ferromagnetic fluctuation is disappeared at TN and this phase below TN is antiferromagnetic single phase in which the magnetic moments of Mn ions lie in ab plane. In general, ab plane of nearly half doped perovskite manganite shows metallic conductivity in A-type antiferromagnetic phase.[17] However, as shown in Fig.1 (c), NdBaMn2 O6 shows a metal-insulator transition at TM I not only along c-axis but also within ab plane. The complicated magnetic phase could be one of the origin of the three dimensional insulating conductivity below TM I .

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28th International Conference on Low Temperature Physics (LT28) IOP Conf. Series: Journal of Physics: Conf. Series 969 (2018) 1234567890 ‘’“”012103

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IOP Publishing doi:10.1088/1742-6596/969/1/012103

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Figure 3. Magnetic field dependence of magnetization below TM I . The numbers denote the measurement sequence. 0.15

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Finally, we show magnetic phase diagram 0.10 of NdBaMn2 O6 which are concluded from the study. Though ferromagnetic fluctuation in 0.05 paramagnetic phase steeply increase just below 0.00 310 K(TP ), long-range ferromagnetic ordering is not realized above 290 K (TM I ). At 150 K -0.05 H // c TM I , magnetic susceptibility suddenly drops due to antiferromagnetic fluctuation. However, -0.10 Hc just below TM I , long-range antiferromagnetic -0.15 ordering is not realized due in part to the -60 -40 -20 0 20 40 60 remaining ferromagnetic fluctuation. The H (kOe) ferromagnetic fluctuation decreases with a decrease of temperature, and disappears at 235 Figure 4. Magnetic field dependence of K (TN ). Below TN , A-type antiferromagnetic magnetization at 150 K (< T ). N single phase is realized. 4. Conclusion Through the measurements of the magnetic field dependence of magnetization around TM I in detail, the magnetic phases of NdBaMn2 O6 are identified. Below 310 K, ferromagnetic fluctuation in paramagnetic phase grows as it gets closer to TM I . On the other hand, the

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28th International Conference on Low Temperature Physics (LT28) IOP Conf. Series: Journal of Physics: Conf. Series 969 (2018) 1234567890 ‘’“”012103

IOP Publishing doi:10.1088/1742-6596/969/1/012103

Ferromagnetic fluctuation in antiferromagnetically fluctuating domains Ferromagnetic fluctuation in paramagnetic phase

A-type antiferromagnetic (S ^ c)

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T(K) Figure 5. Magnetic phase diagram of NdBaMn2 O6 .S is Mn spin moments vector. ferromagnetic fluctuation is observed in the temperature range between TM I and TN . The metamagnetic phase transition which is caused by melting the antiferromagnetic domains is observed by relatively low magnetic field. Because these magnetic phases are strongly coupled with electric conduction properties, a large magnetoresistance effect is expected in this temperature range. Acknowledgments This study was partly supported by a Grant-in-Aid for Scientific Research (No. 24540380) from the Japan Society for the Promotion of Science and by the grant for Strategic Research Promotion(No.G2503) of Yokohama City University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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